The present invention relates to a printer, a copying machine or the like of a laser beam, thermal transfer, ink jet or another type, and more particularly to an image forming apparatus such as a color printer or a color copying machine for recording a plural color image.
Hitherto, a variety of printers and copying machines of a laser beam type, a thermal transfer type, an ink jet type and so forth have been suggested. A major portion of the printers and copying machines of the foregoing types are digital apparatuses arranged such that its recording means is scanned in the main scanning direction and its recording medium is scanned in the sub-scanning direction to two-dimensionally form an image.
In particular, color printers and color copying machines usually have an arrangement that the foregoing operations are performed plural times for a plurality of colors to form a multi-color image.
Although the printers and copying machines of the foregoing type are able to operate preferably, there arises the following problems when a multi-color image is formed.
In a case where a multi-color image is formed by superimposing mono-color images on a recording medium, slight deviation of the image recording position for each color cannot be prevented. Therefore, color shade and moire fringes can be generated undesirably due to change in the state of color mixture. In order to prevent the foregoing problems, a method has been employed in which each color image has an individual screen angle.
However, a digital recording method having a reference recording period of about 400 DPI encounters a fact that the screen angle and the number of screen lines, which can be formed, are limited. What is worse, undesirable difference in the number of screen lines for each color results in generation of a peculiar texture pattern. As a result, a satisfactory image quality cannot be obtained.